This invention relates to an improved semiconductor device having a metal wiring formed on a semiconductor wafer including a device or devices. More particularly, it relates to the metal wiring in the semiconductor device adapted to prevent shorting across the wiring caused by hillocks or whiskers that are produced in the multi-layer metal wiring.
With increase in the degree of integration in semiconductor devices such as LSI's or VLSI's, the need for high density packaging technology has also increased. Such higher density packaging is required for increasing the degree of integration and reducing the power consumption as well as the spacing between adjacent wiring layers. With a view to higher density packaging, the tendency is towards using a multi-layer wiring such as twin or three layer wiring. The problem encountered in providing such multi-layer wiring is the formation of needle-like single crystals, called hillocks or whiskers, that are formed in the lower (first or second) wiring layers. For realizing an optimum insulation between the layers, these hillocks or whiskers must be extremely small and be of the order of about 0.1 to 0.5 .mu.m.
So far, an Al or Al-Cu alloy has been used as wiring metal for a semiconductor device having a multilayer wiring, especially as substrate wiring metal, chiefly by reason of the good adherence of the Al or alloy to an insulating film such as silicon dioxide, and because of the excellent mechanical strength of the resulting wiring. Besides, vacuum deposition and control of film thickness may be made very easily and the overall resistance of the wiring may be reduced with an Al or Al-Cu alloy having higher electrical conductivity. Among the methods known in the art to effect vacuum deposition of aluminium are the Al deposition method by resistance heating, an E-gun evaporation method by melting the aluminium with an electron beam, and a sputtering method by argon sputtering. In the resistance heating, referred to above, an aluminium wire is placed on a filament made of wolfram or tantalum and the filament is heated for melting the aluminium to produce an Al vapor which is deposited in vacuum. In this method, various impurities were mixed into the deposited aluminium film, because the wolfram or tantalum filament has a lower purity than that of aluminium. With the E-gun method, aluminium is melted inside the crucible by means of an electron beam. However, the crucible material is not be expected to be of high quality. In the resistance heating or E-gun methods, impurities exist in the Al film in such a large amount that the problem of whisker formation has not presented itself so seriously.
A high-quality aluminium film, obtained by the sputtering method which uses argon sputtering, has also been used for wiring, for possibly advoiding the adverse effects that the impurities contained in the filament itself or in the crucible material might have on the semiconductor device. With such Al film, the effects of the impurities on the semiconductor device may be reduced, however, with the increased purity in the Al film, the formation of hillocks and whiskers has posed a new problem. For example, with the use of a high-quality Al film as a metal wiring, large hillocks (1 to 2 .mu.m) and whiskers (1.5 to 5 .mu.m) are produced during the processing of the wiring and the subsequent wafer process due to repeated heat cycles with accompanying compression and expansion of the material. With the interval between Al wiring layers and the thickness of the insulation film between the first and second wiring layers in LSI's being in general 2 to 3 .mu.m and 1 to 2 .mu.m, respectively, even supposing that the hillocks and whiskers are covered by the insulating film, these small projections may not be covered sufficiently by the resist material used for providing through-holes in the interlayer film. Due to pinholes thus produced, the whiskers or hillocks may cause electrical shorting between adjacent layers, resulting in the lower yield rate and reliability of the semiconductor device.
Such tendency may be evident not only with a metal wiring made of aluminium, but with one made of high purity wolfram or tantalum or an alloy based on these metals. In other words, whiskers tend to be produced more easily with an increase in the purity of wiring metal.
In order to cope with the problem of whisker formation, a so-called Al-Si alloy with the Si contents being about 1 to 2 percent has been used as a multi-layer metal wiring. However, at the time of an aluminium etching following the photoengraving, aluminium may be removed by etching, but Si remains as a residue. The Si which has remained as a residue may be dispersed quickly in the Al during Al-Si deposition and segregated in the boundary region between the Al-Si alloy and the substrate (insulating film). Excessive Si residues may cause shorting between adjacent wiring and layers thus imparing the protective function or coverage of the insulating films between the adjacent layers. Thus, when the Al-Si alloy is used in the multi-layer wiring, Si residues on the first layer of the Al-Si alloy must be removed by dry Si etching following Al etchings. This is not practically desirable because not only an additional step is involved in the manufacture process, but an SiN insulating film, subjacent to the Al-Si alloy, is also removed by etching simultaneously with Si etching.